Asymmetry of phospholipid distribution. Phosphatidylcholine (PC) and sphyngomyelin (SM) cholinophospholipids are primarily located in the external leaflet of the cell membrane (ECM). Phosphatidylserine (PS) and phosphatidylethanolamine (PE) aminophospholipids as well as phosphatidylinositol (PI) phospholipid are primarily located in the internal leaflet of the cell membrane (ICM). In mammalians, the asymmetry of phospholipid distribution is maintained by the flip/flop movement of phospholipids. PS and PE are maintained to the inner leaflet of the membrane bilayer by the Mg2+-ATPase (i.e. aminophospholipid translocase), and the ATP-dependent aminophospholipid-specific flippase and cholinophospholipid-specific floppase (i.e. multidrug resistance proteins). In pathophysiological conditions, PS and PE are shuttled to the outer leaflet after inactivation of the aminophospholipid translocase or flippase, and activation of the Ca2+-dependent ATP-independent scramblase.